As a result of the increasing electrification of the drive train of modern motor vehicles, semiconductors, in particular in the form of switch elements such as transistors, are used to an increasing extent here. These are exposed to the inhospitable external conditions of the engine compartment, for example passive temperature lifts. Passive temperature lifts are created by the waste heat of the internal combustion engine heating the engine compartment while the vehicle is driving.
A particularly high load acts on the semiconductors of the converter of the electric machine in such motor vehicles, which experience additional loads from active temperature lifts due to the self-heating at high current. Large current amplitudes are typically reached during motor-related boost processes and generator-related recuperation processes (in vehicles having correspondingly operable electric machines). The semiconductors of the converter are functionally critical components since their failure results in a non-functionality of the complete electric machine.
Since the number and the amplitude of boost and recuperation processes are highly dependent on the particular application, i.e., among other things, on the driving behavior of the driver (aggressive, defensive) and/or the traffic situations in which a corresponding motor vehicle is predominantly moved (city traffic, freeway), the configuration of the semiconductors in the converter represents a technical challenge.
To avoid a premature and uncontrolled failure of semiconductors, method for estimating their residual service life may be used. With the aid of a thermal model, the junction temperature is calculated in real time, and the residual service life is determined by storing temperature lifts of the junction temperature. When a drop below a certain residual service life occurs, a preventive maintenance measure is initiated, for example.
Since operating conditions are not always predictable and a plurality of new operating modes exists in modern motor vehicles, the load from active temperature lifts at times varies drastically as a function of the particular application (see above) and the demand placed on the vehicle electrical system (so-called varying field load). This results in also drastically varying residual service lives, which may not always be reliably estimated using conventional methods or to which not always a satisfactory response is possible, even when correctly estimated.
Thus, a need exists for improved options for operating electrified motor vehicles, for example of hybrid or electric vehicles (HEV, EV), which include appropriate semiconductor switches in their vehicle electrical system, and with the aid of which a reliable operation without unexpected failures and comfort losses is made possible.